The event that led to defeat at Waterloo was:
B.a Russian army strikes the final blow at the battle of Waterloo
Explanation:
The Russian army's final strike at Waterloo against the immortal guards of Napoleon which were supposed to be the old guard that never fell and then they were routed from the war leaving Napoleon himself defenseless.
This route was the final blow at the end of Waterloo's battle dealt by the Russians against Napoleon's forces and it resulted in an utter defeat on his side which Napoleon has rarely seen in his life's entire campaigns.
Hii the answer would be C
Hope I helped
